In order to observe a microscopic region of an object, a scanning electron microscope (SEM) or transmission electron microscope (TEM) is employed. In general, in these devices, a second housing in which a specimen is disposed is evacuated in order to bring a specimen atmosphere to a vacuum state, and the specimen is then imaged. There is an intense need for observation of a specimen, which is damaged with vacuum or has the state thereof changed with vacuum, such as a biochemical specimen or liquid specimen using an electron microscope. In recent years, an SEM permitting observation of an object-of-observation specimen under the atmospheric pressure, a specimen retention device, or the like has been developed.
The foregoing devices have, in principle, a thin film or a microscopic through hole, which can transmit an electron beam, disposed between an electron optical system and a specimen so as to partition a vacuum state and atmospheric state. The devices have in common the thin film disposed between the specimen and electron optical system.
For example, patent literature 1 (Japanese Unexamined Patent Application Publication No. 2009-245944) has disclosed an invention of an environmental cell in which a specimen to be observed is stored in a petri dish-like cylindrical container having an aperture, through which an electron beam is passed, formed in the top side thereof, the cylindrical container is put in a second housing of an SEM, a hose is joined with the cylindrical container from outside the second housing, and the interior of the container can thus be sustained in the air atmosphere on a pseudo basis. Herein, the “pseudo basis” signifies that when the interior of the second housing is evacuated, since a gas flows out through the aperture, observation is not strictly performed under an atmospheric-pressure environment.
Patent literature 2 (Japanese Unexamined Patent Application Publication No. 2007-294365) has disclosed an atmospheric pressure SEM having an electron source side of an electron optical lens barrel oriented downward and an objective lens side oriented upward, and having a thin film, which can transmit an electron beam, formed over an exit hole for the electron beam at the distal end of the electron optical lens barrel with an O ring between them. According to an invention described in the literature, an object-of-observation specimen is placed directly on the thin film, a primary electron beam is irradiated from the undersurface of the specimen, and reflected electrons or secondary electrons are detected for SEM observation. The specimen is retained using an annular member disposed around the thin film. The atmospheric pressure SEM preferable for observation of, especially, a liquid specimen is realized by the invention disclosed in the patent literature 2.
Patent literature 3 (Japanese Unexamined Patent Application Publication No. 2007-188821) has disclosed an invention of a small-sized SEM, which permits SEM observation without use of a second housing by retaining a compact electron optical lens barrel in a shroud and overlaying the shroud undersurface with a vacuum seal material so as to bring the entire shroud into close contact with an object of observation, other than an invention of an atmospheric pressure SEM.